1. Field of the Invention
This invention relates to a magnetic field detection apparatus and a method of adjusting the apparatus, or in particular to a magnetic field detection apparatus having a magnetoresistance effect element of spin valve structure and a method of adjusting the apparatus.
2. Description of the Background Art
A method of detecting the angle position and the rotational speed of a rotary unit such as a motor based on the variation of the magnetic field of a magnet arranged on the surface of the rotary unit is known. The conventional magnetic field detection apparatus including a Hall element or a magnetoresistance effect element is generally used as a means for detecting a magnetic field.
The Hall element utilizes the Hall effect in which when a DC current is supplied along the long side of a thin film such as a semiconductor or the like and a magnetic field is applied from the direction perpendicular to the long side of the thin film, a voltage is generated in the direction along the plane of the semiconductor. The magnetoresistance effect element, on the other hand, utilizes the MR (magnetoresistance) effect in which the resistance value is changed under the effect of a magnetic field. In the case where a high detection accuracy is required, a magnetic field detection apparatus including a magnetoresistance effect element is generally used.
The magnetoresistance effect element includes an anisotropic magnetoresistance effect element (hereinafter sometimes referred to as the AMR element) utilizing the anisotropic magnetoresistance effect of a ferromagnetic material, a giant magnetoresistance effect element (hereinafter sometimes referred to as the GMR element) for developing a large resistance change rate with a stack structure of a ferromagnetic member and a nonmagnetic member, and a tunnel magnetoresistance effect element (hereinafter sometimes referred to as the TMR element) for generating the tunnel effect.
Japanese Patent Laying-Open No. 2004-069546, for example, discloses a magnetic field detection apparatus having a GMR element and detecting a mobile unit rotated in synchronism with a rotary shaft. This magnetic field detection apparatus disclosed in Japanese Patent Laying-Open No. 2004-069546 is so configured as not to cross the zero magnetic field by applying a bias magnetic field to the GMR element.
In recent years, a GMR element and a TMR element of spin valve structure having a larger resistance change rate and capable of realizing a high detection accuracy have been proposed. The spin valve structure has a stack of layers including a free layer with the direction of magnetization thereof changing with an external magnetic field and a pin layer with the direction of magnetization thereof fixed without regard to an external magnetic field.
Japanese Patent Laying-Open No. 04-358310, for example, discloses a magnetoresistance sensor having a spin valve structure. Also, Japanese Patent Laying-Open No. 2001-217478 discloses a TMR element having spin valve structure. Further, Japanese Patent Laying-Open No. 11-298063 discloses a TMR element of spin valve structure with the free layer implemented using a permanent magnet layer.
A detection apparatus using these magnetoresistance effect elements having the spin valve structure has also been proposed. Japanese Patent Laying-Open No. 08-226960, for example, discloses a Wheatstone bridge circuit formed of four GMR elements having the spin valve structure. Also, Japanese Patent Laying-Open No. 2003-215145 discloses a rotational speed detection apparatus using the TMR element having the spin valve structure.
As shown in FIG. 2 of Japanese Patent Laying-Open No. 08-226960, the magnetoresistance effect element of spin valve structure has a linear characteristic area generating a resistance value substantially proportional to the external magnetic field and a saturated area generating a predetermined resistance value regardless of the external magnetic field.
Referring to FIG. 39, the resistance value R of the magnetoresistance effect element is expressed as:R=Rm+ΔR/2×Hex/Hk (linear area: −Hk≦Hex≦Hk)R=Rm+ΔR/2=Rmax (saturated area: Hex≧Hk)R=Rm−ΔR/2=Rmin (saturated area: Hex≦−Hk)
where Hex is the external magnetic field and Hk is the magnitude of the saturated magnetic field of the magnetoresistance effect element.
Generally, the detection apparatus using a magnetoresistance effect element of spin valve structure cannot accurately detect a magnetic field out of a linear characteristic area, and therefore the range of detecting the external magnetic field is limited to the magnitude of the saturated magnetic field.
In detecting a minuscule AC component from an external magnetic field containing a large DC component, therefore, the problem is posed that once the DC component exceeds the magnitude of the saturated magnetic field and the magnetoresistance effect element becomes a saturated state, a minuscule AC component cannot be detected.
Further, the magnitude of the saturated magnetic field is determined by the physical characteristics such as the material, thickness and shape of the free layer. Therefore, design and fabrication of the magnetoresistance effect element for various applications requires considerable labor and increases the cost.